Conventional lift control systems utilize hydraulic pumps and cylinders to lift a load at a plurality of different positions. It is desirable to operate each pump and cylinder so that the load remains level while being raised or lowered. The accuracy of such a system can be affected due to variations in pump flows, cylinder capacities, hose lengths and pump efficiencies, among other factors. A cylinder used in such a system is also referred to as a "ram".
An example of one such lift control system is shown in Lionet U.S. Pat. No. 3,968,730 which discloses a plurality of rams each operated in accordance with a servovalve. A linear variable resistive element associated with each ram provides a position signal. The position signal of pairs of rams are compared to indicate the relative positions of the two rams. The comparison signal is combined with an up or down command to partially open or close a particular servo valve, as required, so that the rams remain synchronized. Such a system is complicated due to the fact that the system must be precisely calibrated in order to accurately convert the resistance value of each resistive element. Moreover, such an apparatus cannot insure that incremental movement of each ram is maintained within a desirable level relative to the others. An added problem is the inability to detect that a ram may have moved downwardly into a support surface when under load.
The present invention is intended to overcome these and other problems associated with control systems for hydraulic cylinders.